Answer:
As defined by the glossary to the Catechism of the Catholic Church, a venial sin is "sin which does not destroy the divine life in the soul, as does mortal sin, though it diminishes and wounds it. Venial sin is the failure to observe necessary moderation, in lesser matters of the moral law, or in grave matters acting without full knowledge or complete consent." See the Catechism, nos. 1855 and 1862.
A mortal sin, as defined by the same glossary, is "a grave infraction of the law of God that destroys the divine life (or sanctifying grace) in the soul of the sinner, constituting a turn away from God. For a sin to be mortal, three conditions must be present: grave matter, full knowledge of the evil of the act, and full consent of the will." See the Catechism, nos. 1855 and 1857.